1. Field of the Invention
The present invention relates to an image forming apparatus and a control method therefor.
2. Description of the Related Art
Hitherto, an electrophotographic image forming apparatus has been known. In the electrophotographic image forming apparatus, laser light modulated based on image data is emitted from a laser light source. A charged photosensitive drum is irradiated and scanned with this laser light. Consequently, an electrostatic latent image is formed on the photosensitive drum. This electrostatic latent image is visualized as a toner image with toner supplied from a development unit. The toner image on the photosensitive drum is transferred onto a sheet (recording paper) by a transfer unit.
Thus, when the photosensitive drum is irradiated with laser light based on image data of 1 line, a scanning line of the laser light is ideally a straight line parallel to the direction of an axis of the photosensitive drum. However, actually, sometimes, the scanning line is not a straight line parallel to the direction of the axis of the photosensitive drum due to mechanical errors of the mounting positions of the laser light source, the photosensitive drum and the like. That is, sometimes, the scanning line curves.
In order to correct the curve of the scanning line, for example, Japanese Patent Application Laid-Open No. 2003-241131 has discussed the following first method. Japanese Patent Application Laid-Open No. 2004-170755 discusses the following second method.
According to the first method, in a step of assembling an exposure unit to an image forming apparatus body, the magnitude of the slope of the scanning line is measured using an optical sensor. In addition, the exposure unit is mechanically inclined. Thus, the slope of the scanning line is adjusted. Then, the exposure unit is assembled to the image forming apparatus body.
According to the second method, the magnitudes of the slope and the curve of the scanning line are measured using an optical sensor. Then, bitmap data is corrected to cancel the slope and the curve of the scanning line. Consequently, an image is formed based on the corrected bitmap data.
The second method is to correct the curve of the scanning line and restrain occurrence of misregistration by electrically processing image data. Thus, the second method can eliminate necessity for a mechanical adjustment member and an adjustment step at assembly. Consequently, as compared with the above first method, the second method can restrain occurrence of misregistration at low cost.
However, according to the above second method, line buffers are used to perform blend processing on a curved scanning line to record data of an adjacent line thereabove or therebelow. Necessary capacity of each line buffer depends on the width of the curved part of the scanning line. For example, when the width of the curved part of the scanning line spans N lines of an image represented by bitmap data, line buffers whose total capacity is sufficient to store bitmap data of N lines are necessary. The number N of the spanned lines varies with the apparatus. The total capacity of the line buffers actually mounted in the apparatus should correspond to the number of the lines, which exceeds the maximum value of such variation in the number N of the spanned lines. Consequently, the total capacity of the line buffers increases and the size of a circuit for correcting the bitmap image data increases, which results in increase in cost of the apparatus.